The formation of ceramic shapes from ceramic powders normally involves pressing or extrusion of a mixture of the ceramic powders and various additives so that a dense cohesive structure is formed which is then heated in a kiln (fired) to destroy any residual organic components and to promote the sintering of the inorganic powder so that further densification and strengthening results. It is essential to produce a final structure with minimal free voids between ceramic powder particles in order to achieve the highest possible density and tensile strengths of the green and sintered bodies. Although pressure of formation plays an important role in densification, pressing under high loads (&gt;100 MPa) will not convert ceramic powder alone into a firm body. The shape formed in this manner would simply crumble when ejected from the die or mold. Organic binders are used to overcome these problems. Organic binders provide lubricity to facilitate the compression of the green structure into a high density form, and adhesion so that the "green", unfired, body holds together. Upon firing and the onset of sintering, the organic binder is no longer required. Complete pyrolysis of the organic material present, so that there are no residues which might adversely affect the sintering process, occurs in the early stages of the firing process.
In practice, the ceramic powder, e.g., alumina, is dispersed in a liquid carrier (water or organic) with the aid of chemical dispersants and mechanical action. A dispersant is necessary in order to make a stable, liquid dispersion at high solids. Although a variety of materials may be used for this function, frequently a low molecular weight poly(acrylic acid) as its sodium or ammonium salt is used. The level of use of this dispersant is usually in the range of 0.05-0.5% by weight based on alumina. Other dispersants that have been used are poly(methacrylic acid) salts and lignosulfonic acid salts. An organic polymeric binder and other functional materials such as lubricants and sintering aids are then added. The resulting slurry or slip is spray dried to yield a free-flowing powder consisting of spherical agglomerates (granules) of about 50-200 micrometer diameter. Approximately 2-8% binder (based on the dry weight of powder) is commonly used. The formulated powder can next be pressed to the desired shape in a suitable die from which it is then ejected.
The viscosity of the complete slurry must be suitable for necessary handling and spray drying. Although spray dry equipment and running conditions may be adjusted to handle a variety of viscosities, larger particles will result from higher viscosity slurries. The resultant large particles may result in larger interstices between particles and hence a lower strength. The binder may contribute to viscosity of the continuous phase of the slurry by virtue of its molecular weight, solubility, conformation in solution, and possible incompatibility with the combination of powder and dispersant. The spray-dried blend of powder and binder must be free flowing so that it can completely fill dies.
The resulting compacted part must be smoothly ejected, be as dense as possible, and not suffer significant dimensional change from that of the die. Chemical additives have a major effect on the desired lubricity. Polyethylene oxides and fatty acid derivatives promote lubrication (the former may also behave as a binder). During dry pressing, the granules are deformed and the binder flows to fill available space, thus increasing density. The glass transition temperature (Tg) of the polymer can have a strong effect in this step. Polymers with too high a glass transition temperature will not flow and as a result cohesion of the pressed part does not occur. Under these conditions, the compressed powder will undergo stress relaxation in the form of expansion on release from the die. This phenomenon is referred to as "springback" and is undesirable from the standpoint of dimensional accuracy as well as density and strength. For this reason, plasticizers are used with higher Tg polymers. Frequently, mixtures of polymers having greatly different physical properties are used as binders. For example, a plastic material such as polyethylene oxide may be blended with a film-forming binder such as polyvinyl alcohol to give an effective ceramic binder.
The polymer binders currently used for the binding of ceramic powders have, as a class, not been specifically synthesized for optimal densification and green strength of ceramic forms. Rather, they are commercially available materials, with other principal uses, which have been adapted for use in ceramic manufacture and include such materials as cellulose ethers, polysaccharides, polyacrylic latexes, poly(2-ethyl-2-oxazoline), poly(ethylene oxide), poly(vinyl alcohol), poly(vinyl butyral), and wax. Of these, poly(2-ethyl-2-oxazoline), poly(ethylene oxide), poly(vinyl alcohol), and wax are used in the spray drying process.
The principal function of the binder is to hold the compacted form together after pressing. The method utilized for determination of suitable "green strength" is the diametral compression strength or DCS of a cylindrical section across its diameter. DCS is actually a measure of tensile strength. The unit of measurement of the pressure tolerance is the megapascal (MPa). Typical values for DCS of "green" parts are in the range 0.3-3.0 MPa. It should be recognized that the strength of the green part is largely a function of the state of the binder phase. If the binder does not flow (as in the case of too high a Tg), or if it is a liquid (as in the case of too low a Tg), a weak non-coherent green part will result. The binder must coat/wet the ceramic powder, flow easily at the pressure and temperature of the die, and be cohesive at the test temperature.
Binders must be cost effective and highly soluble in water or suitable organic solvents, such as alcohols, toluene, or xylene.
Oxide ceramics absorb atmospheric moisture. Since water is known to plasticize alumina and many hydrophilic organics, alumina green bodies will lose strength when exposed to humid conditions (R. A. Dihilia et al, Advan. Ceram. 9, 38-46 (1984); C. E. Scott et al, Amer. Ceram. Soc. Bull. 61 (5), 579-581 (1982). Binders which would have less sensitivity to moisture, without loss of cohesive strength, would clearly be advantageous.
In summary, a preferred binder for ceramic powders would be soluble in water and a wide range of organic solvents. This ideal binder would be suitable for all ceramic powders and form easily spray-dried mixtures, be compatible with all dispersants and even function itself as a dispersant. It would allow pressing to maximum density with minimal pressure and eject easily from the die. Springback would not occur. It would not require a separate plasticizer or be sensitive to atmospheric moisture. The ideal binder would not leave any deleterious residues during the burn out and firing.
Although poly(ethylene oxides) or poly(ethylene glycols) are commonly used as a binder for ceramics in industry, they have some deficiencies. The use of poly(ethylene oxides) is generally limited to aqueous processes where they produce slurries of high viscosity. They have solubility only in methanol and methylene chloride at ambient temperature. These two solvents are very low boiling and toxic.
Poly(2-ethyl-2-oxazoline) is more expensive than the polyethylene glycol-type resins. It has as an advantage a low molecular weight which gives rise to low aqueous system viscosities. A deficiency of this resin is its high Tg (70 C) which is too high for normal processing and must therefore be admixed with a plasticizer (PEG 400). This polyether however renders poly(2-ethyl-2-oxazoline) formulations hygroscopic. Poly(vinyl alcohol) also requires plasticizing.
The various acrylic latex type binders produce very desirable low viscosity slips. The dried powders, however, are very hard and do not press well.
Aluminum oxide (alumina) is by far the most widely used powder in technical ceramics. Zirconium oxide (zirconia), beryllium oxide (beryllia), and other oxides are used in special applications as are the non-oxides, silicon carbide and silicon nitride.